What some of you have missed is this - having belts (white/yellow/green etc..etc..) can help maintain some students as it rewards progress.
But in Aikido it seems to be an issue due to the natural of Aikido - togeetherness and the like. We should all blend and not be above each other so the blet "system" and Aikido seem to clash.
Also, and I believe this to be very important is there is probably (in the eyes of someone whose been around a while) little difference between a blue belt and a blue belt with two green stripes, if you do that sort of thing - so I'm saying that belts, if you have them need to have real value.

In BJJ for instance we start at white, move to blue, purple, brown and black. Think of this - a BJJ Blue belt is a proverabial light year better than a white belt, it is a measurable and extremly tangible thing, the first step to blue is huge and they will make absolute mince meat of a white belt, but they are no match for a purple belt. Achieveing a brown belt is considered a remarkable feat, and I can say that of the 130 students at our BJJ club almost all of them would see a black belt as unatainable. A belt system can work well, if there is real value in the belt.

Having said all this I don't like it in Aikido as it clashes with the philosophy of it all, brown belts are fine, but under that just be happy to train.